This invention relates in general to electronic fluorescent display devices and in particular, to a low voltage cathodoluminescent device particularly useful for flat mosaic large-screen and ultra-large screen full color hang-on-wall type displays.
Cathode ray tubes (CRT) have been used for display purposes in general, such as in conventional television systems. The conventional CRT systems are bulky primarily because depth is necessary for an electron gun and an electron deflection system. In many applications, it is preferable to use flat display systems in which the bulk of the display is reduced. In U.S. Pat. No. 3,935,500 to Oess et al., for example, a flat CRT system is proposed where a deflection control structure is employed between a number of cathodes and anodes. The structure has a number of holes through which electron beams may pass with a set of X-Y deflection electrodes associated with each hole. The deflection control structure defined by Oess et al. is commonly known as a mesh-type structure. While the mesh-type structure is easy to manufacture, such structures are expensive to make, particularly in the case of large structures.
Mosaic large-screen full color displays have been used frequently in public environments such as sports stadiums and exhibition halls. Several types of mosaic full color large-screen displays have been in use or proposed. In one type known as the flat matrix CRT, its anode voltage is as high as 8 kilovolts or higher and has low phosphor dot density. It is mainly used in the outdoor environment. Because of the above-described characteristics, it is difficult to construct a thin, high dot density display for use in indoor applications such as for hang-on-wall televisions using the flat matrix CRT.
Another conventional system currently used is known as the Jumbotron such as that described in Japanese Patent Publication Nos. 62-150638, 62-52846. The structure of Jumbotron is somewhat similar to the flat matrix CRT described above. Again the anode voltage is as high as 8 kilovolts or above and the display panel at least over 1 inch in thickness. Each anode includes only less than 20 pixels so that it is difficult to construct a high phosphor dot density type display system using the Jumbotron structure.
Both the flat matrix CRT and Jumbotron structures are somewhat similar in principle to the flat CRT system described by Oess et al. discussed above. These structures amount to no more than enclosing a number of individually controlled electron guns within a panel, each gun equipped with its own grid electrodes for controlling the X-Y addressing and/or brightness of the display. In all the above-described devices, the control grid electrodes used are in the form of mesh structures. These mesh structures are typically constructed using photo-etching by etching holes in a conductive plate. The electron beams originating from the cathodes of the electron guns then pass through these holes in the mesh structure to reach a phosphor material at the anodes. As noted above, mesh structures are expensive to manufacture and it is difficult to construct large mesh structures. For this reason, each cathode has its own dedicated mesh structure for controlling the electron beam originating from the cathode. Since the electron beam must go through the hole in the mesh structure, a large number of electrons originating from the cathode will travel not through the hole, but lost to the solid part of the structure to become grid current so that only a small portion of the electrons will be able to escape through the hole and reach the phosphor material at the anode. For this reason the osmotic coefficient, defined as the ratio of the area of the hole to the area of the mesh structure of the cathode, of the above-described devices is quite low.
To counteract the low osmotic coefficient and also to increase phosphor brightness in these devices, high voltages are used such as 8 kilovolts or above. To prevent undesirable arcing, it is therefore necessary to increase the distance between the anode and cathode, thereby resulting in a thick display device. Furthermore, since each cathode has its own dedicated mesh structure, in order to avoid mutual interference between adjacent mesh structures, it is necessary to leave sufficient spacing between the mesh structures of adjacent cathodes. For this reason, each display panel in the above-described devices includes less than 20 pixels so that it is difficult to construct a high phosphor dot density type display system using the above-proposed structures.
Another conventional mosaic full color large-screen display system is the color vacuum fluorescent display such as that described in Japanese Patent Publication No. 62-52836. It employs a cathode, an anode, and one grid. An auxiliary cathode and light leader are used to increase dot density. The anode voltage used is around 300 volts. The anode and grid are used for X-Y addressing. Since the anode is used in addressing, the anode voltage cannot be higher than 300 volts in order to prevent electrical shorts between anodes. However, the luminescence of the three primary colors red, blue and green (R, B and G) phosphors are low at voltages such as 300 volts and below. Furthermore, at such voltages, the phosphors have short lifetimes.
In the above-described three types of mosaic full screen displays, complex electronic circuitry is required which takes up considerable space behind the display. The face plate of the systems used in these devices are thick so that it is difficult to construct high density and thin devices which can be used as hang-on-wall televisions.
Yet another conventional mosaic full color large-screen structure that has been used is back lighting liquid crystal displays (LCD). Its structure has many thin film transistors R, B, G photoarrays so that it is difficult and expensive to manufacture. A large number of lighting sources need to be used behind the display screen and only a small portion of the light from the light sources is transmitted so that it is inefficient.
In all conventional mosaic displays constructed using a two-dimensional array of panels, there will be mosaic slots between the panels. These slots would appear as dark square or rectangular grid lines superimposed onto the displayed image and affects the quality of the displayed image. For back lighting LCD displays, the mosaic slots are relatively large which degrades the display image. Due to the large number of lighting sources behind the screen, these LCD devices are generally over 2 feet in thickness. It is therefore difficult to use the back lighting LCD in large-screen hang-on-wall television systems. Thus even though the back lighting LCD has high resolution, it has not been widely used.